Adult diet and breast cancer has been extensively studied but few clear associations have emerged. Yet adulthood may not be the most susceptible time period during a woman's life time for dietary exposures to affect breast cancer initiation or progression. Emerging evidence supports the importance of experiences during early life in shaping breast cancer risk, in particular, prior to menopause. Mammary tissue develops during adolescence but terminal structures of the mammary gland do not differentiate until the first pregnancy. Mathematical models of breast cancer etiology also demonstrate that the years before first birth are critical in establishing breast cancer risk. Challenges in collectng relevant data on early life and adolescent diet, however, have limited the number of studies on adolescent diet and breast cancer risk. Understanding the role of diet in the etiology of breast cancer is of great importance since diet is a potentially modifiable risk factor. Dietary pattern analyses allow us to examine diet as a whole and to move beyond considering the individual role of foods and nutrients in disease risk. We propose to use already collected data on adolescent nutrition and premenopausal breast cancer incidence in the Nurses' Health Study II. Participants completed a food frequency questionnaire about their diet during high school in 1998. Dietary patterns will be generated using factor analysis and reduced rank regression. We propose to conduct a prospective analysis using a Cox proportional hazards model to estimate hazard ratios of breast cancer incidence associated with each of the dietary patterns. We will restrict our study to premenopausal women since pre- and postmenopausal breast cancer have different risk factor profiles. Our aim is to identify adolescent dietary patterns and investigate whether these dietary patterns are related to premenopausal breast cancer risk: a prudent (healthy) pattern to a lower incidence and a Western pattern to a higher incidence of premenopausal breast cancer. Furthermore, we will examine whether women with an adolescent dietary pattern that is associated with markers of insulin resistance and inflammation have a higher incidence of estrogen-receptor negative premenopausal breast cancer. We have a unique resource available since our dietary data have been collected prior to breast cancer diagnosis and we have prospectively collected data on many important covariates, including a large number of known risk factors for breast cancer. The results of this study may inform prevention strategies and shift the focus of breast cancer prevention efforts to this critical perid of susceptibility.